Re: [PATCH-for-6.2 0/2] hw/display: Do not allow multiple (identical) VGA devices

[Philippe Mathieu-Daudé]

On 11/19/21 10:58, Thomas Huth wrote:
> On 19/11/2021 10.49, Philippe Mathieu-Daudé wrote:
>> On 11/19/21 09:21, Mark Cave-Ayland wrote:
>>> On 18/11/2021 19:20, Philippe Mathieu-Daudé wrote:
>>>
>>>> Commit 7852a77f598 fixed creating multiple TYPE_ISA_VGA devices,
>>>> generalize the fix to all VGA devices.
>>>>
>>>> See https://gitlab.com/qemu-project/qemu/-/issues/44
>>>>
>>>> Philippe Mathieu-Daudé (2):
>>>>     hw/display: Add Error* handle to vga_common_init()
>>>>     hw/display: Do not allow multiple identical VGA devices
>>>>
>>>>    hw/display/vga_int.h        |  2 +-
>>>>    hw/display/ati.c            |  4 +++-
>>>>    hw/display/cirrus_vga.c     |  4 +++-
>>>>    hw/display/cirrus_vga_isa.c |  4 +++-
>>>>    hw/display/qxl.c            |  4 +++-
>>>>    hw/display/vga-isa-mm.c     |  3 ++-
>>>>    hw/display/vga-isa.c        | 11 ++---------
>>>>    hw/display/vga-pci.c        |  8 ++++++--
>>>>    hw/display/vga.c            | 17 ++++++++++++++++-
>>>>    hw/display/virtio-vga.c     |  4 +++-
>>>>    hw/display/vmware_vga.c     |  2 +-
>>>>    11 files changed, 43 insertions(+), 20 deletions(-)
>>>
>>> Hi Phil,
>>>
>>> I don't think this is correct for non-ISA devices: for example years ago
>>> I had a PC running Windows 98SE with 2 identical PCI graphics cards
>>> configured in dual-head mode.
>>>
>>> IIRC the BIOS would bring up the first graphics card and configure it to
>>> use the legacy ISA VGA ioports for compatibility, and then once the main
>>> OS drivers loaded both cards were switched to PCI mode and configured
>>> using the BARs as normal.
>>
>> The problem here is QEMU technical debt, not the hardware.
>>
>> When vga_common_init() calls memory_region_init_ram_nomigrate()
>> with obj=NULL, "vga.vram" is registered as a QOM singleton.
>>
>> Updating it would
>> 1/ require non-QOM devices to be QOM'ified
> 
> So sounds like that's the right way to go here.
> 
>> and 2/ break migration unless using HPFM which I don't master.
> 
> What's HPFM?

Hocus Pocus Freakin' Magic